It is known, as disclosed in U.S. Pat. No. 3,395,261, to join two overlapped thermoplastic materials together by heating, by induction, a susceptor material that in turn melts the thermoplastic materials which can then be pressed together to form a bond. Prior art devices for accomplishing this, as disclosed in such patent, have included the use of a pair of high frequency current carrying metal rollers to apply heat and pressure to the thermoplastic sheet materials to be joined and to inductively heat a susceptor material of iron oxide particles placed between the sheets of thermoplastic material that are heated and then bonded by the generalized heat thus created and by the pressure of the wheels which may be teflon coated to avoid sticking.
The prior art method discussed above has drawbacks in that the rollers, which are metal and conductive, themselves become heated and cause transfer of heat by conduction to the material to be joined. Sticking of the heated metal rollers to the thermoplastic material is also a problem requiring teflon coating of the pressure rollers for them to be effective. Moreover, the device described in the patent requires two contacts by induction sources on both sides of the relatively thin sheets of thermoplastic material that are being joined.
During induction heating of thermoplastic composites (i.e., thermoplastic materials reinforced with non plastic materials such as glass or carbon fibers) it has been observed that substantial deconsolidation of the laminate may occur which, in the case of reinforcement with susceptor fibers may result in arcing between the spaced fibers. On the other hand, a fully consolidated joint is one where there are substantially no voids between the fibers.
It is also known, according to European Patent Application No. 88301739.4 that pressurizing exposed surfaces of an induction heated laminated thermoplastic joint overcomes the problem of delamination of such surfaces by pressurizing during or immediately subsequent to the welding operation by means of pressure pads or rollers mounted in a fixed relationship in the vicinity of the induction coil, but on the opposite side with the joint to be welded being moved between the two in a fixed direction. This apparatus had the drawback of producing uneven edge heating depending on susceptor fiber orientation and required access to both sides of the joint as well as limiting application of pressure on the weld joint to a single direction.
As a solution to the problems of the above described apparatus, it was proposed in an Edison Welding Institute Progress Report dated May 17, 1988, that a welding tool consisting of an induction coil mounted inside a pressure application roller and containing an after-roller for reconsolidation would solve the delamination problem. In the proposed apparatus, an idler roller was used to drive the after roller at the same linear speed as the main roller. However, this apparatus also had its drawbacks resulting from the difficulty of moving the tool in other than a straight line and the continued application of heat and wave effect created by the smaller after rollers if reverse passes were necessary to form a bonded joint.